Two huge explosions rocked the Boston Marathon finish line at Copley Square Monday afternoon, killing three and causing scores of casualties, including several traumatic amputations on streets crowded with runners, spectators and post-race partiers.

The number injured has soared to nearly 150 — including three children suffering from lower leg injuries, hospital officials tell the Boston Herald.

“We will get to the bottom of this. We will find out who did this and we will find out why,” President Barack Obama said Monday evening. “They will feel the full weight of justice.

“Boston is a tough and resilient town. Residents will pull together, take care of each other and move forward,” he added. “The American people are with them every step of the way.”

“We’ve had a horrific attack in Boston this afternoon,” Patrick said at a news conference, where police confirmed two explosions at the finish line at 2:50 p.m. EDT and a third fire or “incendiary device” at the John F. Kennedy Library, where black smoke was seen billowing.

“This is a tragedy,” Menino said. “We’re going to work together on this.” He added he gives his “condolences and prayers” to the families of the injured and killed runners.

Boston Police Commissioner Edward Davis said no suspect has been apprehended, but numerous people are being interviewed.

Massachusetts General Hospital, with 22 patients, reported that six were critical, including four with traumatic amputations. Boston Medical Center reported 20 injured with two children among the wounded there. Their conditions were not immediately available. Tufts Medical and Brigham and Women’s Hospital also reported injured and well as Beth Israel Hospital — bringing the total to at least 100.

Herald reporter Chris Cassidy, who was running in the marathon, said, “I saw two explosions. The first one was beyond the finish line. I heard a loud bang and I saw smoke rising. I kept running and I heard behind me a loud bang. It looked like it was in a trash can or something. That one was in front of Abe and Louie’s. There are people who have been hit with debris, people with bloody foreheads.”

“Somebody’s leg flew by my head,” said a spectator who gave his name as John Ross. “I gave my belt to stop the blood.”

People were yelling, “I need my kids!”

“It was horrific!” said a man who gave his name as Brian Walker. “I saw some horrific wounds. You could literally feel the rush of wind.”

Trevor Finney described “panic” and said he saw a teen girl apparently missing a leg being carried away.

City Council President Steve Murphy, who was at the finish line when the two explosions happened, said, “Police sources say they are finding more devices.”

Police have discovered at least two other devices and had a controlled detonation on at least one object.

Panicked families have begun gathering at local hospital.

“I need to see my mother!” one woman screamed to uniformed security holding media back at bay outside Brigham and Woman’s Hospital.

A frantic scene was unfolding at Boston Medical Center, where a parade of ambulances have been arriving at the emergency room entrance, where gurneys and wheelchairs were lined up in awaiting casualties, some of whom arrived wrapped in foil survival blankets commonly given out to runners at the finish line to keep warm.

A BMC spokesman called it a “developing situation” and said hospital officials don’t yet have a full picture of what their patient toll may be.

Supervisory Special Agent Martin Feely of the FBI’s New York Bureau said there was no advance warning or hint of a planned attack on Boston Monday.

“Not at all,” Feely said.

Feely could neither confirm nor deny that New York has already dispatched agents to assist their Boston brethren, but said, “To the extent that assistance is needed, we certainly will.”

Federal agents and police have begun converging on the crowded hospitals, with ATF and FBI agents seen at the Brigham and Boston Medical Center, while a bomb unit was seen at Tufts Medical Center.

MBTA spokesman Joe Pesaturo said subway service has been suspended on the Green Line between Kenmore and the Park Street stations. Runners were being told to move on to the Boston Common. People were being instructed to turn off their cell phones as multiple calls began jamming cell phone service. The Prudential Center was evacuated.

Patrick, who was at the marathon finish line hours before the explosion, was back at the State House Monday afternoon working with his staff in the aftermath of the events.

“We don’t have any good information yet,” he said. In a statement shortly afterward, the governor said: “This is a horrific day in Boston. My thoughts and prayers are with those who have been injured. I have been in touch with the President, Mayor Menino and our public safety leaders. Our focus is on making sure that the area around Copley Square is safe and secured. I am asking everyone to stay away from Copley Square and let the first responders do their jobs.”

Patrick said that staffers are headed back to the State House.

A White House officials said: “Shortly after being notified of the incident around 3 p.m. EDT, the President received a briefing from Homeland Security adviser Lisa Monaco and other members of his senior White House staff in the Oval Office. The President called Boston Mayor Tom Menino and Massachusetts Governor Deval Patrick to express his concern for those who were injured and to make clear that his administration is ready to provide needed support as they respond to the incident.”

Athletic shoemaker New Balance was in the midst of hosting its marathon viewing party at the Solas Irish pub at the race’s finish line in the base of the Lenox Hotel on Boylston Street, when the explosion occurred across the street.

“Everyone here is okay,” a spokeswoman said in text messages sent from Solas. “The building shook, but the hotel staff made people stay inside, and we are all safe. You can smell smoke, and people are now getting carted off. It’s bad. It was right next to the finish line.”